A centrifuge is a piece of equipment for rotating a specimen at a high speed. Such high speed rotation generates a centrifugal force and causes denser substances of the specimen to separate out along the radial direction. The centrifuge is employed in the areas of biology, physics, medicine, chemistry, etc.
The centrifuge includes a rotor driven by a motor and rotating at a high speed. There are various types of rotors including vertical rotors, hanging rotors, fixed angle rotors, etc.
The subject of this patent application is a fixed angle rotor, which comprises a plurality of slot radially placed around a center of rotation center and slanted at a certain angle. The slots receive a tube containing a specimen and may have a various of sizes and positions in the rotor depending on their application fields. The slots are rotated at a high speed and, therefore, a high centrifugal force is generated, which causes the substances of the specimen to be separated based on their density.
Such high speed rotation generates a stress distribution according to a centrifugal force in the rotor as well as the specimen. The centrifugal force per unit volume is proportional to the square of rotational speed, distance and density. As a result, if the rotor is fabricated from materials having high specific strength, that is, having low density and high strength, it can rotate at a higher speed. Therefore, various attempts are tried to increase the rotational speed of the rotor by employing composite materials, which generally have high specific strength.
Based on the position of use of composite materials, conventional composite material centrifuge rotors may be classified as three types, a metal inner portion—a composite material outer portion, a polymer inner portion—a composite material outer portion, and a composite material inner portion—a composite material outer portion.
U.S. Pat. No. 5,057,071 discloses a centrifuge rotor including an aluminum inner portion and a composite material outer portion. U.S. Pat. No. 4,824,429 discloses a centrifuge rotor including a polymer inner portion and a composite material outer portion. However, the latter has the problem that stress concentration is generated around the slots of the polymer inner portion. U.S. Pat. Nos. 5,643,168, 5,759,592, 5,776,400 and 5,362,301 disclose a rotor including a composite material inner portion and outer portion. KR application no. 10-2010-0019254 discloses a light weight fixed angle hybrid centrifuge rotor.
When a rotor includes a composite material inner portion and outer portion, as obviously described in each of the above patents, the inner portion is reinforced with fiber along a radial direction (r) and the outer portion is reinforced with fiber along a peripheral direction (θ). In other words, even though various arrangements comprising a quasi-isotropic, random, or weaving arrangement are employed in the inner portion, the inner portion is basically formed by stacking such various arrangements in r and θ planes along the axial direction, that is, the vertical direction (z-direction) of a circular cylindrical coordinate system, and the outer portion surrounds the inner portion by arranging fiber along the peripheral direction (θ).
The basic principle of such a structure is such that the expansion of a rotor, which is generated when the rotor rotates at a high speed, is suppressed by reinforcing the inner portion with fiber along the radial direction and the outer portion with fiber along the peripheral direction.
As shown in FIG. 1, stress concentration is generated around slots, which causes serious problems when designing a centrifuge rotor. Taking a symmetrical distribution of the slots into consideration, stress analysis about only one slot was performed. The result of the stress analysis shown in FIG. 1 indicates that stress is concentrated around the slot along the radius and peripheral directions.
In a way to relieve such stress concentration, as shown in FIG. 3, conventional centrifuge rotors employ a reinforcer of composite material inserted into a slot, in which fiber is arranged along the peripheral direction of the slot mainly. FIG. 2 shows Von Mises stress distributions in an isotropic inner portion depending on the presence or not of the reinforcer, and indicates that the maximum stress around the slot decreases by 37%.
In the case of the conventional rotors described above, the reinforcer of composite material surrounded by the inner portion has an effect to decrease the stress concentration generated in the rotors. However, since such decrease is not sufficient, the reinforcer and parts of the inner portion around slots still remain as fragile parts.